Reviews and extends the theory of Lie groups, develops differential geometry, proposing compact definitions of torsion and of curvature, and adapts the usual notion of linear tangent application to the intrinsic point of view proposed for physics. Uses a unifying illustration: two simple theories are studied with some detail, the theory of heat conduction and the theory of linear elastic media. Shows that the resulting equations derived in this manner differ quantitatively and qualitatively from those usually presented.